Tbi fuel pressure

[VIKING_MECHANIC]

So I know what the spec is for tbi fuel pressure but I can't find any information on how long it's supposed to hold pressure after the key is off.

I suspect the check valve is/has failed and is letting the fuel drain back into the tank way sooner than it's supposed to be.

The truck usually has a pretty long crank time and today I cycled the key about 4 times and it immediately started.

 

[GoToGuy]

What year, make , model, eng ? It makes a difference.

 

[Ace17]

I can say that for mine, it only held pressure for a few seconds. I actually put an adjustable fuel pressure regulator on, along with a gauge. If you turn the key on, you can hear the fuel pump come on for a few seconds, and then it shuts off, along with the fuel pressure, until you turn the key into the start/cranking position.

I've got an 88 k1500 btw.

 

[VIKING_MECHANIC]

What year, make , model, eng ? It makes a difference.

89 K1500, 5.7

 

[Scooterwrench]

There is a tiny bleed hole in the top of the regulator that I'm guessing is there to bleed off any trapped air back to the return side of the regulator and it will bleed off the pressure once the pump shuts off. I don't think there is a check valve in the pump so pressure will bleed off back through the pump once it shuts off also.

 

[PlayingWithTBI]

There isn't a check valve in a TBI setup. The flat diaphragm (I don't have a picture of it) returns whatever it needs in order to maintain pressure while the pump is running. Once you shut off the pump, your pressure will drop immediately.


Red arrow = Fuel supply, Blue arrow = Fuel return, Green T = Pressurized fuel to the injectors.

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[VIKING_MECHANIC]

There isn't a check valve in a TBI setup. The flat diaphragm (I don't have a picture of it) returns whatever it needs in order to maintain pressure while the pump is running. Once you shut off the pump, your pressure will drop immediately.


Red arrow = Fuel supply, Blue arrow = Fuel return, Green T = Pressurized fuel to the injectors.

You must be registered for see images attach

Okay but if I just immediately turn the key on, it will have a what feels like a labored crank before it finally starts(10+seconds). If I cycle the key several times, it will immediately start.

 

[VIKING_MECHANIC]

But shouldn't it build pressure and hold it for a few seconds with key on before it depressurized ?

 

[PlayingWithTBI]

Okay but if I just immediately turn the key on, it will have a what feels like a labored crank before it finally starts(10+seconds)

IIRC there is a setting in the .bin which will delay starting when cold or, your Fuel Pump relay isn't working and your running off the Oil Pressure Switch.

But shouldn't it build pressure and hold it for a few seconds with key on before it depressurized ?

Yes, the pump will run for 2 seconds with KOEO, then shut off and instantly bleed the system.

 

[Ace17]

Yeah this could be a myriad of things. Like PlayingWithTBI mentioned, if you're not building oil pressure quickly enough, or the oil pressure switch is faulty, it could either delay starting, or not start at all.

Personally I would look at things like dirty fuel filter, fuel pump, maybe even clogged injectors. Could even be bad fuel. Does it do this often, or has it happened only recently? Last time you changed your fuel filter? How does the truck run when you're driving it? Is it perfectly fine, or does it feel anemic?

It sounds like it just isn't getting enough fuel to light off on the initial crank. I don't know if you've done tuning or have changed anything, but if you've made significant changes(cam, heads, intake, etc.) and not did a tune, the truck could run pretty weird.